SpecProposalDeadline (SPD) is an optional step in the release cycle: a deadline before which all specs for a given cycle should have been filed for review. It generally happens before the second development milestone.
Once SPD kicks in, any new spec proposed won't be considered for inclusion in the current release cycle. It may be considered and accepted for the next release cycle.
SPD ensures that specs are filed early enough to allow for timely review. It also acts as a precursor to SpecApprovalDeadline.
If you want to propose a spec for a minor, desired feature after SPD, you may do so by raising a specific topic to the corresponding project team meeting. Exceptions may be granted by the PTL of the affected project.